Juba , South Sudan -LRB- CNN -RRB- -- A large group of youths from an ethnic militia loyal to former South Sudanese Vice President Riek Machar is marching for the city of Bor , just days after it was retaken by the government amid heavy fighting , officials said .

Col. Phillip Aguer , spokesman for the Sudan People 's Liberation Army , estimated 20,000 ethnic Nuer from the so-called White Army were headed for the city , raising fears of more violence .

The U.N. Mission in South Sudan said Sunday it is `` extremely concerned '' about the report , but it could not confirm the size or location of the marchers .

`` South Sudan does not need another escalation of the crisis involving armed youth , pitching communities against communities , '' said Hilde F. Johnson , the U.N. special representative of the secretary-general .

The U.N. mission warned that the involvement of armed youths `` would add a volatile and unpredictable ingredient into the precarious security situation currently prevailing in the country . ''

The youths -- known for the white powder they use to cover their skin as an insect repellant -- do not have formal military training and , if not under direct orders , can act without discipline .

Information Minister Michael Makuei said Saturday that rebel leader Machar had gone to ethnic Nuer neighborhoods in Uror to mobilize the youths .

Multiple media outlets reported Machar denied the youths are acting under his orders . CNN has not been able to contact Machar 's representatives .

The same group is believed responsible for the deaths of hundreds of civilians from the Murle ethnic group in 2011 and 2012 .

Machar is a longtime rival of President Salva Kiir . The men are from two different tribal clans . Kiir is from the Dinka tribe , Machar from the Neur .

Kiir accused Machar of trying to stage a coup . Its ' a claim Machar has denied .

Fighting broke out between Kiir 's government and supporters of Machar on December 15 in the capital city of Juba . It quickly spread across the country , with reports of mass killings lent credence by mass graves .

Kiir 's soldiers regained control of Bor last week in a bloody battle for the city .

A growing chorus of international agencies is calling for an end to the hostilities .

East African leaders on Friday gave the warring factions four days to lay down their arms .

If they do n't , the leaders of the Intergovernmental Authority on Development warned they 'll `` take action '' to stop the conflict that the United Nations says has killed more than 1,000 and forced some 121,000 from their homes .

The warning came the same day the United Nations said the first of 5,500 additional peacekeepers had arrived in the country .

The humanitarian crisis here is worsening , with at least 122,000 displaced since the fighting began . Roughly 63,000 have sought refuge in U.N. bases in the country , according to the world body .

Those on the U.N. base in Juba are scared to venture outside , even though many have homes within walking distance .

James Gajaak , the de facto community leader at the Juba U.N. camp , told CNN 's Arwa Damon that when he first arrived with his family , he went three days without food and water .

`` So my brother went back to the house to collect water , '' he said , recalling how later he received a phone call . `` They tell me , ` Your brother is dead ; come collect the dead body so that you can bury him . ' ''

Doctors from the relief group Doctors Without Borders are concerned over the increasing volume of people within the confined space of the camps .

Christine Bimansha is one of many doctors providing medical care at two UNMISS sites for Doctors Without Borders -LRB- Medecins Sans Frontieres -RRB- . She said in an MSF field report Sunday that she is concerned about sanitation conditions and the potential spread of disease .

`` Water and sanitation is the biggest problem the displaced people are facing -- they do n't have access to water or to enough latrines . If the water and sanitation conditions at the UNMISS camp remain the same , the situation will get worse -- if people do n't have water and we have an explosion of diseases like cholera , then that will be a big outbreak and a big problem . Measles is also a significant threat and an outbreak would be difficult to stop in such overcrowded conditions . ''

South Sudan 's neighbors threaten to step in to end fighting

CNN 's Antonia Mortensen reported from Juba , South Sudan . Neda Farshbaf reported from Atlanta . CNN 's Sarah Aarthun contributed to this report .

@highlight

Youths from an ethnic militia are marching for Bor , days after the government retook the city

@highlight

The youths are loyal to rebel leader Riek Machar

@highlight

Machar denies reports they are acting under his orders

@highlight

The U.N. says it is `` extremely concerned '' by the reports